1. Field of the Invention
The present invention relates to an engine exhaust gas purification device.
2. Description of the Related Art
If a ratio of the total amount of air fed into the exhaust passage upstream of a certain position in the exhaust passage, the combustion chamber, and the intake passage to the total amount of fuel is referred to as an air-fuel ratio of the exhaust gas at that certain position, there is known an engine in which a lean air-fuel mixture is burned in the combustion chamber, a three way catalyst is arranged in the exhaust passage, and a NOx absorbent, which absorbs NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is lean and releases absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is rich, is arranged in the exhaust passage downstream of the three way catalyst (see PCT international publication WO93/07363).
Namely, in this engine, the amount of production of NOx becomes maximum when an air-fuel mixture which is slightly leaner than the stoichiometric air-fuel ratio is burned, and the amount of production of NOx becomes smaller as the air-fuel ratio is made leaner or richer with respect to the air-fuel ratio of this air-fuel mixture. In addition, when a lean air-fuel mixture is burned, the amount of unburned HC and CO discharged from the engine becomes considerably small, as compared with the case where a rich air-fuel mixture is burned. Accordingly, in the above-mentioned engine, the air-fuel ratio of the air-fuel mixture to be burned in the combustion chamber is made considerably lean to reduce the amount of NOx and unburned HC and CO discharged from the engine as much as possible. In addition, at this time, the NOx discharged from the engine is caused to be absorbed by the NOx absorbent as much as possible, and the unburned HC and CO discharged from the engine are caused to be oxidized by the three way catalyst as much as possible to reduce the amount of NOx and unburned HC and CO discharged into the outside air.
Among the harmful components in the exhaust gas, NOx is a component which is the most difficult to deal with. If this NOx can be reduced by the three way catalyst, it is possible to considerably lower the amount of NOx in the exhaust gas. However, if the air-fuel ratio of the exhaust gas is lean, the three way catalyst does not carry out the reducing operation of NOx. Accordingly, if the air-fuel ratio of air-fuel mixture is made lean, i.e., the air-fuel ratio of the exhaust gas is made lean as in the above-mentioned engine, NOx discharged from the engine is not reduced by the three way catalyst and, accordingly, in the above-mentioned engine, NOx passes through the three way catalyst without being reduced and then flows into the NOx absorbent.
Namely, in the above-mentioned engine, the first stage of the reducing operation of the amount of NOx is carried out by making the air-fuel ratio of the air-fuel mixture lean. In this case, it is true that the amount of NOx discharged from the engine is reduced due to this first stage of the reducing operation of the amount of NOx, but the absolute amount of NOx discharged from the engine is still large. Although NOx discharged from the engine flows into the three way catalyst, the amount of NOx is not reduced in the three way catalyst at all. The NOx flows into the NOx absorbent, and the second stage of the reducing operation of the amount of NOx is carried out therein by the absorbing operation of NOx by the NOx absorbent.
In this way, in this engine, the reducing operation of NOx is carried out in two stages, but the main reducing operation of the amount of NOx relies on the absorbing operation of NOx by the NOx absorbent. However, even if the NOx absorbent is used, particularly when the absolute amount of NOx flowing into the NOx absorbent is large, all the NOx cannot always be absorbed in the NOx absorbent. Accordingly, it is difficult to make the amount of NOx, discharged into the outside air, almost zero as long as the main reducing operation of NOx is made to rely on only the absorbing operation of NOx by the NOx absorbent.